2. Review Literature 2.1 Anatomy and development of the elbow joint The elbow joint (cubital joint) is a stable compound ginglymus or hinge joint: It is composed of the humero-radial joint formed by the humeral condyle and head of the radius, the humero-ulnar joint, which is formed by the semilunar notch of the ulna and humeral condyle, and the proximal radio-ulnar joint (FOX et al., 1983; LEWIS et al., 1989; NICKEL et al., 1992; EVANS, 1993; VOLLMERHAUS et al., 1994). The proximal radio-ulnar joint is composed of the articular circumference of the radial head with the radial notch of the ulna (LEWIS et al., 1989). All components share a common joint capsule. The elbow joint enables flexion, extension and a limited amount of rotation. The humero-radial joint is responsible for 75% to 80% of weight-bearing surface of the joint (BERZON and QUICK, 1980) while the humero-ulnar part stabilizes and restricts the movement of the joint in the sagittal plane and the proximal radio-ulnar joint allows rotation of the antebrachium (BERZON and QUICK, 1980; FOX et al., 1983; EVANS, 1993) The anconeal process of the ulna articulates with the caudal intercondylar surface of the humerus and fits into the supratrochlear fossa when the joint is fully extended. The trochlear notch of the ulna articulates with the trochlear of the humerus. Distal to the trochlear notch are two prominences called the medial and lateral coronoid processes. The medial process is larger and is located more distal than the lateral (BERZON and QUICK, 1980; FOX et al., 1983). Both prominences are articular and act to increase the 8 total surface area of the elbow joint without contributing to its weight-bearing function (BERZON and QUICK, 1980). The medial prominence or the medial coronoid process (MCP) and the lateral prominence or the lateral coronoid process (LCP) constitute 20% to 25% of the total articular and weight-bearing surface (LJUNGGREN et al., 1966). The radial head articulates with a curved depression between the two coronoid processes called the radial notch and is held in place by the annular ligament (MILLER et al., 1964). The lateral (ulnar) collateral ligament originates on the lateral epicondyle and after blending with the annular ligament divides distally into two crura. The larger cranial portion inserts onto the radial head, while the thinner caudal part inserts in the ulna. A sesamoid bone is occasionally found between the ligament and the radial head (BAUM and ZIETZSCHMANN, 1936). The smaller and weaker medial (radial) collateral ligament originates on the medial epicondyle and also divides into two crura. The weaker cranial part attaches onto the radial head, while the stronger caudal part passes deeply into the interosseous space where it attaches not only to the ulna, but also to the radius (EVANS, 1993). The medial collateral ligament prevents abduction of the elbow joint and the lateral collateral ligament prevents adduction of the elbow joint (GORING and BLOOMBERG, 1983). The annular ligament of the radius is a thin band that runs transversely around the radius. It originates beneath the lateral collateral ligament at the base of LCP and inserts on and below the MCP as it blends with the medial collateral ligament (MILLER et al., 1964). The annular ligament is essential for maintenance of normal articulation between the humerus and the radius and ulna (MILLER et al., 1964; LJUNGGREN et al., 1966). The MCP makes direct contact with the lateral articular surface of the humeral condyle during normal movement of the elbow joint (MILLER et al., 1964; TIRGARI, 1974). Any aberration that would affect 9 the integrity of these articular surfaces could be a source of severe discomfort (BERZON and QUICK, 1980) Several separate ossification centers are involved in the development of the elbow joint (HARE, 1961). In the immature elbow, there are six growth plates (BOULAY, 1998). The humeral condyle is formed by two secondary ossification centers: the medial and lateral condyles, which ultimately fuse and possess epicondyle (HARE, 1961; LEWIS et al., 1989). The proximal radius is configured by one secondary ossification center. The proximal ulna, in most instances, has two ossification centers and the anconeal process, which can be radiographically recognized, is formed by one secondary ossification center, in particular, the olecranon apophysis (BOULAY, 1989; MORGAN, 2000). The ossification centers of the anconeal process appear at 12 to 14 weeks and may develop as a direct extension of the diaphysis of ulna or as a separate center of ossification (HARE, 1961; VAN SICKLE, 1966; OLSSON, 1983; GUTHRIE, 1992; TURNER et al., 1998), and at this time, the cartilaginous medial coronoid process begins to ossify from base to its tip and has no separate center of ossification (FOX et al., 1983; OLSSON, 1983; FLÜCKIGER, 1992; GUTHRIE, 1992; BREIT et al., 2004). The ossification of the coronoid process and the fusion of the anconeal process are complete by approximately 16 to 22 weeks (HARE, 1961). In the German Shepherd dog and other large breed dogs the fusion of the anconeal center to the ulna occurs most commonly between the ages of 16 to 20 weeks (VAN SICKLE, 1966; SCHRÖDER, 1978; FOX and WALKER, 1993; SJÖSTRÖM, 1998; TURNER et al., 1998), but in the greyhound between 14 to 15 weeks (VAN SICKLE, 1966). If the anconeal process is not radiographically united at 20 weeks of age, spontaneous union will not occur (FOX et al., 1983; FEHR and MEYER-LINDENBERG, 1992; FOX and WALKER, 1993). We 10 can see complete ossification of the coronoid process by radiography at the age 20 to 22 weeks (OLSSON, 1983). In some dogs of these breeds, a separate center of ossification may be present in the anconeal process, which unites with the olecranon between the ages of 4 to 6 months. Ossification of the centers of growth occurs early. The medial and lateral condyles of the humerus are first needed to be ossified at 2 to 4 weeks of age and the head of the radius at 3 to 5 weeks of age. The medial epicondyle ossifies later at 6 to 9 weeks. The olecranon ossifies at 7 to 9 weeks (MORGAN, 2000). WOLSCHRIJN and WEIJS (2004) used microcomputer tomography to evaluate the trabecular alignment within the medial coronoid process and specify the direction of forces within the bone during development. Primary trabecular alignment was found to be perpendicular to the humero-ulnar articular surface in dogs aged four to 24 weeks. This direction is the same as the direction of forces produced by the humero-ulnar joint during weight-bearing. Secondary cranio-caudal alignment corresponding to stresses from the annular ligaments was identified at 13 weeks of age, whereas KÜNZEL et al.(2004) studied the subchondral split line patterns of canine medial coronoid process in bones obtained from 26 deceased large-breed dogs, and determined three main types of split line patterns; the sagittal type, the transverse type, and the intermediate type. These three types corresponded well with the fissure and fragmentation line patterns of the MCP. 11 Fig. 1 Split line (a–c) and fragmentation line patterns (d–f) seen in canine ulnae. Note the sagittal alignment of the split lines (black arrow) in (a) characterizing the sagittal type split line was aligned in parallel to the lateral border and at right angles to the rim of the tip and medial border of the MCP. The alignment (black arrows) of split lines in (b) characterizing the transverse type split lines were orientated in a transverse line to both collateral borders (c) representing the intermediate transition type between sagittal and transverse type as the split lines were aligned obliquely to the longitudinal axis of the MCP . Also note similarities in the alignment of the split lines in (a) with the course of the sagittal fragmentation line (white arrow) in (d) showing a left ulna of a male Rottweiler aged 10 years (proximal projection), of the split lines in (b) with the course of the transverse fragmentation line (white arrow) in (e) illustrating the left ulna of a female French bulldog aged 10 years (craniomedial projection), and of the split lines in (c) with the course of the multiple fragmentation lines (arrows) in (f) seen in the left ulna of a male Rough Collie aged 1 year (cranioproximal projection). AP, anconeal process; MCP, medial coronoid process; Rad, radius (KÜNZEL,2004) 2.2 Biomechanics of the elbow joint The elbow joint is composed of 3 joints; humero-radial, humero-ulnar and proximal radio-ulnar. The humero-radial joint and proximal radio-ulnar joint are simple kinematical joints because they only have one articulation in each joint. The humero- ulnar joint has two contact articulations between the humeral trochlea and the trochlear notch of ulna (THOMSEN et al., 2001). VAN HERPEN (1988) and EVANS (1993) described the three joints of the elbow joint with functions independent of each other. The humero-radial joint carries the bulk of the weight; the humero-ulnar joint causes the strict interaction, and the proximal radio-ulnar joint allows the rotation of the forearm. LIPPERT (1990) also divided the elbow joint of the human being into its three functional part-joints: The humero-ulnar joint is a uniaxial hinge and executes inflection and stretching. The proximal radio-ulnar joint is also a uniaxial joint and executes 12 bicycle movement: inwards and outwards. The humero-radial joint is a two-axial joint with hinge and rotations, whilst the humero-ulnar joint can rotate only a little (MONTAVON and SALVODELLI, 1995). The lateral collateral ligament and medial collateral ligaments are strongly developed and essentially contribute to the strict interaction within this joint (EVANS, 1993).